A vibratory roller machine of the type to which this invention relates has a roller drum by which compacting forces are imposed upon a surface traversed by the machine and has an exciter housed in the drum by which rapidly alternating up and down acceleration forces are imposed upon the drum to enhance its compacting effectiveness. The exciter ordinarily comprises a rotating eccentric mass, and its direction of rotation must correspond to the direction in which the roller is moving along the surface.
It is important that the exciter should be stopped at any time that the roller is not moving. If the exciter were allowed to operate with the machine at a standstill, the roller would tend to pound an objectionable groove into the surface on which it was resting; but, more important, the vibration produced by the exciter would ruin the bearings of the eccentric and of the drum by a process called brinelling.
The exciter can comprise a hydraulic motor that is controlled by a three position valve actuated by a pair of solenoids, one of which is energized for forward rotation of the exciter and the other of which is energized for its reverse rotation. When neither solenoid is energized, no pressure fluid flows to the exciter motor and the exciter does not operate.
One familiar type of drive for effecting movement of the whole vibratory roller machine comprises a variable displacement hydraulic pump that can be controlled by a single manually shiftable member such as a lever. In a neutral position of that control member, the pump delivers no pressure fluid output and the machine stands still. If the control member is shifted through a range of forward positions at one side of the neutral position, the pressure fluid output of the pump is such that the machine moves forwardly at a speed which corresponds to the distance by which the member is displaced from the neutral position. In like manner, moving the control member through a range of reverse positions at the opposite side of the neutral position causes the machine to move rearward at a speed corresponding to displacement of the control member from neutral.
Heretofore, vibratory rollers have been equipped with cam switches or the like that were actuated by the control member which governed the direction and speed of the machine. The cam actuated switches were electrically connected with the valve solenoids for the exciter, to control their energization in correspondence with the position of the control member and thus automatically coordinate operation of the vibratory exciter with the movements of the machine. In prior control mechanisms of this type, the cam-actuated switch for each valve solenoid had to be closed as the control member was shifted through a range of adjustments at each side of neutral but had to be open when the lever was in its neutral position. If the exciter was to be stopped only when the machine was at an absolute standstill, the cam-actuated switches had to be very carefully and critically adjusted, and their adjustment had to be maintained at all times, notwithstanding the intense pounding vibration to which the entire machine was subjected during most of the time that it was in operation. Since it was not feasible to establish and maintain such a precise adjustment for each switch, the cam-actuated switches were usually so adjusted as to cause the exciter to be stopped whenever the control number was near its neutral position and in a position at which the machine moved slowly forward or backward, in order to ensure that the exciter would not be operating when the machine was stopped.
Such an arrangement was satisfactory if the machine always operated in open areas where it could move safely at relatively fast speeds.
However, there are situations in which it is desirable to operate a vibratory roller in a confined space or relatively close to an obstruction, or in which, for some other reason, it is desirable to have the exciter in operation while the roller is moving very slowly in its forward or rearward direction. For such situations it should not be necessary to make the critical adjustment of cam-actuated exciter control switches that has heretofore been necessary.